The present invention relates to recovering hydrocarbons from a subterranean reservoir which contains tar or very viscous oil and is relatively deeply buried. More particularly, the invention provides an improved process for recovering hydrocarbons from a reservoir having a high permeability, but a low fluid mobility, due to a high concentration of tar or viscous oil which is substantially immobile at the reservoir temperature.
In the following, the term "tar" is used to refer to viscous hydrocarbons which may contain gas and/or water, and which, at the temperature of a subterranean reservoir containing them, are substantially immobile.
In at least two locations in the world tremendously large heavy hydrocarbon accumulations are known to exist in relatively highly permeable formations: vis. the Athabasca tar sands in Canada and the Orinoco (Faja) tar sands in Venezuela. At present these hydrocarbons are not produced commercially. In Venezuela, existing producing capacity of lighter, more valuable crude oil already exceeds demand. In the Athabasca tar sands, the reservoir temperature is so low as to reduce the in situ formation fluid mobility to zero (for all practical purposes). The resulting low injectivity prevents application of previously known thermal recovery processes.
Numerous processes have been proposed for recovering relatively immobile oil from subterranean reservoirs, for example, in patents such as the following: U.S. Pat. No. 1,150,655 suggests using an electrical heater in an open borehole in a deposit of carbonaceous minerals for vaporizing and collecting volatile hydrocarbons. U.S. Pat. No. 3,583,488 suggests increasing the uniformity at which steam is injected into a reservoir by uniformly elevating the temperature of a liner to about the steam injection temperature before the steam is injected through the liner. U.S. Pat. No. 3,739,852 suggests heating an oil reservoir by initially injecting steam into the reservoir at less than fracturing pressure, then fracturing the reservoir by injecting steam at a pressure greater than the fracturing pressure, thus forming a fractured and steam-heated zone which is relatively cylindrical, for use in recovering oil by backflowing fluid from that zone. U.S. Pat. No. 3,993,155 suggests improving a process for recovering viscous oil from a reservoir which will not readily accept direct steam injection by initially injecting steam into a well opened into the reservoir while simultaneously outflowing and venting fluid from near the well bottom in a manner such that the injected fluid sweeps any condensed liquid from the well bore, continuing this until the injected steam will enter the reservoir at a reasonable rate without fracturing the reservoir, injecting steam directly into the so heated reservoir, without the simultaneous fluid production, and subsequently recovering oil by backflowing fluid from the reservoir. A series of U.S. Pat. Nos. 4,008,765; 4,019,575 and 4,120,357 relate to processes for preheating reservoir formations around substantially vertical wells by circulating steam through closed loop flow paths which extend to near the bottom of the wells, producing fluid from below the closed loop flow paths and injecting fluid from adjacent wells arranged for driving oil toward the bottom of the wells containing the closed loop flow paths and recovering oil from the produced fluid.